In continuous or semi-continuous casting of metals and metal alloys, a hot melt is supplied to a chilled mould intended for continuous casting, that is, a mould that is open in both ends in the casting direction. The mould is normally water-cooled and surrounded and supported by a supporting structure. Usually, the supporting structure comprises supporting beams or supporting plates provided with inner cavities or channels for a coolant, such as water. The melt is supplied to the mould, whereby the metal solidifies and a cast strand is formed when it passes through the casting mould. When the cast strand passes out of the mould, it comprises a solidified self-supporting shell around a remaining melt.
To prevent the cast strand from adhering to the mould wall, an oscillatory motion is imparted to the mould. To further prevent the solidified self-supporting shell from adhering to the mould wall, a lubricant is usually supplied to the upper surface of the melt in the mould. Through the oscillations, so-called oscillation marks arise on the surface of the cast strand. If the solidified surface layer should adhere to the mould, this manifests itself as considerable surface defects and in certain cases as a ripping of the solidified surface layer.
One known way of preventing the occurrence of oscillation marks on the cast strand is to make use of electromagnetic casting (EMC). During electromagnetic casting, an ac field generates forces acting to separate the melt and the mould and thus reduce the contact pressure between the melt and the mould. Because of these separating forces, the risk of adhesion and the risk of oscillation marks are reduced. Further, improved conditions for lubricating the mould are achieved. In this way, the surface fineness of the finished casting may be improved.
The ac field that is needed during electromagnetic casting is obtained from a coil arranged at the upper end of the mould. This coil may have one or more phases. Preferably, a high-frequency alternating magnetic field is applied. Usually, the inductive coil is fed with an alternating current with a fundamental frequency of 50 Hz or more. For slabs, the frequency is preferably in the interval of 50–1000 Hz, but higher frequencies are feasible. The compressive forces that are generated by the high-frequency magnetic field reduce the pressure between the mould wall and the melt, whereby the conditions for lubrication are considerably improved. The surface quality of the cast strand is improved and the casting speed may be increased without jeopardizing the surface quality. A disadvantage that has occurred in connection with electromagnetic casting is that the induced power losses become very high.
A typical mould for casting of large castings comprises four plates made of copper or a copper alloy which together form a casting mould. These plates are supported by a supporting structure of plates and/or beams. To reduce the inductive power losses, it is known to use stainless steel in this supporting structure, but the power losses are still significant.
Swedish patent document No. 512691 discloses a device for casting of metal, where the power induced in the supporting beams and supporting plates of the mould is reduced, which in turn results in the total induced power losses being reduced. The disclosed device comprises a mould, an induction coil arranged at the upper end of the mould, and a mould supporting structure to mechanically support the mould. The mould comprises a number of mould elements, which are separated by means of partitions, each of which comprises an electrically insulating barrier. Each mould element is associated with a corresponding mechanically supporting mould supporting structure part and an electric conductor with an electrical conductivity that is higher than the electrical conductivity of the supporting structure.
The electric conductor is arranged close to the mould supporting structure part on that side of the mould supporting structure part that faces away from the mould. The barriers in the partitions break the current paths for the electric currents that are induced in the mould by the magnetic field, whereby the penetration of the melt by the magnetic field is facilitated and the induction power losses in the mould are minimized. The electric conductor provides an advantageous return path for the current that is induced by the high-frequency magnetic field, such that the induced power losses are minimized in the supporting structure. Admittedly, this mould arrangement reduces the induced power losses, but still the induced power losses are too high.